1. Field of the Invention
This invention relates generally to the field of radiography with mammography, vascular imaging, and x-ray microscopy being particular examples. More particularly, this invention relates to a new and improved method and apparatus for conducting internal biological imaging, industrial imaging, or microscopic imaging.
2. Description of Related Art
Medical and industrial x-ray imaging typically use a conventional x-ray tube for x-ray production. The spectrum of x-ray energies emitted from an x-ray tube is very broad. However, a much narrower range of x-ray energies is advantageous for many applications. Typically, a conventional film or screen-film system is used for x-ray detection. In some applications, fluorescent screens are used to convert x-ray images to visible light that can be captured with a camera. A relatively new x-ray detection technique is the use of electronic x-ray imaging detectors to obtain digital images. The quality of an x-ray image and the radiation dose required to obtain it are determined by the characteristics of the x-ray detector and the x-ray source working together.
One very common medical procedure is mammography, which is used for the diagnosis of breast cancer. In mammography, radiologists look for small features called microcalcifications. Because the features are small, a higher resolution x-ray film is used as compared to general radiography, and an x-ray tube with a small source size is required to avoid blurring of the image. The quality of the mammography image is degraded by x-ray scattering, and an anti-scatter grid is often required to preserve image quality. A modified mammography procedure for more detailed examination of a smaller area, referred to as cone projection, places even more stringent requirements on the x-ray source size and film resolution. A relatively high x-ray dose is required to resolve small abnormalities.
The diagnostic effectiveness of mammography is limited by the difficulty of distinguishing abnormal features from the features of normal breast tissue. Research has shown that monoenergetic radiation of, for example, approximately 17 to 20 keV, is more effective than broad band radiation for distinguishing abnormal features. Filters are used to reduce the energy spread of radiation from conventional x-ray tubes. Research has also shown that the comparison of x-ray images obtained at two different x-ray energies is effective for distinguishing abnormal features due to different changes in the absorption properties of calcifications and normal tissue at different x-ray energies.
Monoenergetic x-ray radiation, can be generated from several types of sources. Examples of previously considered narrow band x-ray sources include: the use of a Bragg-crystal monochromator, fluorescence x-rays from a secondary target, laser produced x-rays, x-rays produced from Compton backscattering on an electron beam, x-rays excited in a target by a proton beam, channeling radiation, and synchrotron radiation. Such sources have disadvantages that prevent their widespread use for radiography. For example, recent studies have shown that improved radiographic images can be obtained by using the very bright monochromatic radiation generated from synchrotron facilities. However, the present availability of synchrotron facilities, and concomitant cost thereof is far too limiting for general medical use.
One example of a common vascular imaging procedure is coronary angiography, which is used to image the arteries in the heart. In this procedure, a contrast agent (usually an iodine compound) is injected into a blood vessel to make the blood vessel visible in an x-ray image. Unfortunately, this is a dangerous procedure since it requires that a catheter be employed for injection of the contrast agent. The catheter is inserted into a peripheral artery and then threaded up to the heart so that the catheter tip is at the entrance of a coronary artery.
If the need for the arterial catheter in coronary angiography could be eliminated, then x-ray imaging could be done with greatly reduced risk. The risk could be reduced by injecting the contrast agent into a vein. However, the image quality is seriously degraded with a venous injection of the contrast agent because the contrast agent is diluted by about 20:1 by the time it has gone through the lungs and both sides of the heart and starts to fill the coronary arteries. Differential angiography, in which x-ray images acquired at two different x-ray energies are subtracted, is required to resolve the coronary arteries with venous injection of a contrast agent. Differential angiography has been demonstrated with synchrotron radiation, but such facilities are not available or suitable for general use.
X-ray microscopy provides information unobtainable with visible-light microscopy due to the different absorption properties of x-rays and the possibility of imaging smaller structures. As the capabilities of x-ray microscopes improve, they are becoming valuable tools in biology and materials science. High quality x-ray microscope images can be obtained with synchrotron radiation. However, there is a need for smaller and less expensive facilities for x-ray microscopy. One possible solution to this problem is projection x-ray microscopy. However, the performance of such devices is presently limited by the characteristics of available x-ray sources.
In general, it is difficult to realize all the desired features of radiography at the same time. For example, the use of a thicker and larger x-ray detection medium in order to achieve increased detection efficiency generally degrades the image resolution. The use of energy filters with a conventional x-ray tube in order to narrow the energy spectrum of the x rays generally requires a higher power x-ray tube, and undesirable radiation at other energies leaks through the filters. There is a need for a method to obtain x-ray images with monochromatic radiation at dual energies with high spatial resolution, good contrast, high detection efficiency, and low dose. The present invention satisfies these needs.